The invention relates to a molding composition made from a thermoplastic with color dyes and its use for bonding thermoplastics with the aid of laser welding.
Laser welding of thermoplastics has not yet become widely established in industry, since high capital expenditure deters potential users. However, H. Potente et al. in Plastverarbeiter No. 46 (1995), pp. 42-46, describes the enormous advantages of this process in various applications. In particular a considerable potential market is forecast for the jointing process in the future (see Plastverarbeiter No. 48 (1997) pp. 28-30).
In laser welding two plastics are normally combined with one another by bonding an upper plastic translucent to laser light with a lower plastic not translucent to laser light. The laser beam here passes through the upper layer of plastic leaving it unchanged and encounters the lower layer, by which it is absorbed with liberation of thermal energy. The thermal energy liberated melts the plastic material and thus bonds it to the upper layer at the point of impact of the laser beam.
A disadvantage of this method, however, is that it is not possible to process plastic compositions colored with absorbing dyes or pigments or comprising absorbing fillers, since the filler or dye or, respectively, the pigment used for coloration always immediately absorbs the laser light so that no bond is produced.
A particularly important example is that of polyester molding compositions pigmented black using carbon black. Generally, black molding compositions of thermoplastics are colored black with carbon black or other black pigments, but such molding compositions either absorb or reflect light, including laser light, in such an extent so that they cannot be used as the upper layer for laser welding. That means, shaped articles made from such composition cannot be combined with another layer of plastic by means of laser welding.
U.S. Pat. No. 5,540,998 discloses coating composition comprising which were colored black by a combination of colored, non-black pigments instead of black pigments. These compositions comprise constituents which result in curing of the coating, thus yielding a material which is no longer thermoplastic. The presence of pigments in general will cause laser light to be absorbed, reflected or dispersed. Therefore shaped articles colored black by a pigment combination as disclosed in U.S. Pat. No. 5,540,998 are unsuitable as the upper layer for laser welding as well. In general, the presence of mineralic fillers or other compounds that are finely dispersed within in the matrix will mostly cause laser light to be dispersed, absorbed or reflected as well, so the use of colorants and additives with thermoplastic molding compositions used for laser welding is restricted. It is desirable to have colored thermoplastic molding compositions available which are suitable as an upper, at least translucent layer for laser welding.
It is particularly desirable to have thermoplastic molding compositions available which are colored black and which are suitable for this purpose.
The object of the present invention was to provide a black thermoplastic molding composition which is translucent or transparent to laser light and which therefore is suitable for bonding by the laser welding method to other plastic parts comprising absorbing dyes, pigments or fillers.
This object has been achieved by a molding composition comprising a dye combination made from nonabsorbing, polymer-soluble dyes. Surprisingly, the molding composition comprises a combination of non-black, polymer-soluble dyes, which give the molding composition a black color.
It is surprising that this material can be used to bond black plastic parts to colored or black plastic parts.